In 1893, Rudolf Diesel was awarded a patent for the diesel engine. Gandhi committed his first act of civil disobedience. Thomas Edison created the movie studio. And zany New Zealand became the first country to give women the right to vote. Nabisco invented Cream of Wheat.
It was also the year that direct current (DC) took a back seat to alternating current (AC) after Niagara Falls Power Company chose AC transmission for its power plant.
Although we live in an AC-dominated world, DC seems poised for a comeback, particularly in data centers. Facebook adopted a DC architecture in its Prineville, Ore., data center. SAP spent $128,000 retrofitting a data center at its offices in Palo Alto, Calif., to rely on DC power. In 2010 it cut SAP’s energy bills by $24,000 per year.
ABB, the Swiss-Swedish conglomerate, bought a controlling interest last year in Validus DC Systems, which specializes in DC data center equipment. ABB also opened a factory in North Carolina to produce HVDC (high voltage DC) equipment for delivering power from solar and offshore wind farms to the grid. The Tres Amigas “superstation” will rely heavily on HVDC.
Nextek Power Systems and the EM
merge Alliance are also promoting DC as a way to cut power in buildings.
Behind the DC drive
What’s driving it? Although AC became the standard for electronic transmission, DC didn’t disappear. It just hid. Servers, large numbers of electric motors, batteries, even ships and airplanes run on DC. Solar panels produce DC power. Wind turbines can produce AC or DC power, but the extreme variability of wind power means that electricity generated by turbines has to pass through battery banks before it gets to the grid. As a result, wind farms are effectively DC.
To solve the mismatch, a whole industry of AC-DC converters has been developed. National Semiconductor sells billions of dollars’ worth of chips to convert power. Inverters in the solar industry exist to convert DC from solar panels to AC that can run on the wires in your home.
In data centers, the AC-DC gymnastics top the charts. Typically, AC from the grid has to be stepped down in voltage so it can be routed safely in building equipment. Lower-voltage AC then gets converted to DC so it can go to an uninterruptable power supply (UPS). DC power from the UPS then gets converted to AC so it can go over the wires in the building. Then it gets converted back to DC. Usually five conversions, or steps, downward take place.
By converting grid AC at the door of a data center to medium-voltage DC or converting stepped-down AC to DC at the last possible moment, a data center can cut utility bills by 10 to 20 percent or more, according to Trent Waterhouse, the VP of marketing for power electronics at General Electric.
Validus and others have also eliminated some of the technological hurdles involved in transmitting via DC, namely the monster-sized copper cables.
The same dynamics work in buildings. In a retail establishment, DC power from solar panels could go directly to DC-powered LED lights with not-intermediate conversions that sap energy, according to Nextek. Perhaps not coincidentally, Redwood Systems, the lighting networking company, touts that its technology is actually an example of DC networking.
More savings comes in real estate. DC data centers require 25 percent to 40 percent less square footage than their AC counterparts, largely because computer equipment can connect directly to backup batteries.
In a hypothetical example, a 2.5-megawatt data center power module in the AC world might need 7,295 square feet, Ronald Ranaldi, the VP of sales at Validus, told me last year. An equivalent DC power module might occupy only 5,102 square feet, a savings of 2,193 square feet. What’s more, a single data center might consist of several 2.5-megawatt modules.
“Real estate is often greater than the energy savings,” says Ranaldi. “In large, green field data centers, you are literally eliminating buildings.”
DC won’t take over the world. And not everyone is sold. Google is not taking DC for its data centers in part because of the cost that would be involved in retrofitting their existing architecture. But it seems that an idea that was current when Grover Cleveland was in the White House and Japan was just adopting the Gregorian calendar could make a comeback.
Image courtesy of The Planet